It's a level of service above their payment processing, where they also collect and remit certain taxes which I believe is built-in to that ~8%. This is the same service that Google and Apple provide for in-app payments.

https://stripe.com/managed-payments

This is what Apple said about it:

> Stripe and Paddle are leading full-service, turnkey payment processing, and merchant-of-record providers handling all the services necessary for link-out, including payment processing, tax compliance, fraud detection, and customer service. Across the top apps, the mean implied processing and merchant-of-record fee based on Stripe’s and Paddle’s rack rates (which few, if any, large developers actually pay because they negotiate discounts or provision in-house) is 8.1% using Stripe rates, or 7.8% using Paddle rates.

https://storage.courtlistener.com/recap/gov.uscourts.cand.36...